That's something you can do outside of prime95; the schedutil scheduler does that by default, which I believe is why I had hard crashes with ondemand and not schedutil.

(Ok, it actually ramps up frequencies slower rather than lock out cores, but the effect is the same.)

you can probably bodge it by running a bunch of copies with limited workers, then run your main copy, then kill the starters. or, you could fiddle with the CPU affinity.

That's an interesting thought!